This invention relates to pastes, slurries and dispersions of powders in liquids and has particular connection with methods for the safe handling of powdered solids, more especially during the preparation of solutions of such substances.
It is often necessary to measure and control the flow of a powdered solid into a process step. It is known that some powders, in particular those containing a high proportion of fine particles, are readily dispersible in the atmosphere and can give problems of dust formation. This may be a particular problem with toxic substances. It is also known that some powdered solids are susceptible to ignition or detonation. It is further known that some powdered solids are difficult to handle, for example when metering using a screw feeder. It may be convenient to disperse a powder in a suitable liquid to form a paste, slurry or dispersion (all such solid/liquid mixtures being on occasion referred to hereinafter as dispersions) both for ease of storage and transport and to alleviate such problems. The liquid is often and conveniently a process liquid, for example either the solvent (if it is desired to form a solution of the solid) or some other liquid required in or after the process step into which the solid is to be introduced. In other cases, an inert liquid or a volatile liquid which can subsequently be removed from the process by evaporation may be used.
Some dispersions are difficult to form, because the solid is not readily wetted by the liquid. It is known to include wetting agents in a dispersion to alleviate this difficulty. Some dispersions are unstable, in that the solid and liquid phases tend to separate on storage or when the dispersion is subjected to shearing forces, for example during pumping. It is known to include dispersing agents in a dispersion to inhibit such separation. For example, aqueous dispersions of the textile auxiliary 1,3,5-triacryloylhexahydro-1,3,5-triazine (TAHT) which contain a dispersing agent are described by D M Lewis et al. in Journal of the Society of Dyers and Colourists, Volume 111, January/February 1995, pages 12-18.
Nevertheless, there is always the risk that such dispersions may become dried out. This may happen for example with splashes or with residues in pipes and vessels. If this occurs, then any hazards associated with handling the dry powder can recur. It is an object of the invention to provide a means of guarding against such problems.